Heretofore, when the vacuum chamber of an apparatus for forming a thin film on a substrate in which dusts in large quantity may be generated is, for example, evacuated by a vacuum pump, it is known that a mesh member is provided for adhering or collecting the dusts in a gas to be evacuated, the mesh member being interposed in an evacuating passage to protect the vacuum pump from the dusts. It is also known that a drum rotating in oil is provided in the evacuating passage to adhere the dusts in the gas to be evacuated to the surface of the drum or small articles which are contained in the drum.
In an ultrafine particle producing apparatus, produced ultrafine particles are deposited in a collecting chamber to be collected.
When the gas to be evacuated is passed through the mesh member or the drum rotating in oil, Reynolds number is small since the evacuated gas is lower pressure, and the gas flow is laminar. Therefore, the above-mentioned adhesion of fine or ultrafine particles mainly depends on the diffusion effect by the Brownian motion of these particles.
In this case, in order to sufficiently remove the dusts, it is necessary to form the evacuating passage narrow to readily adhere the dusts to the surface of the passage and to flow the evacuated gas while maintaining a required flow rate in the film forming apparatus. As a result, a large pressure difference is required to pass the evacuated gas. However, the pressure difference decreases vacuum suction force of the vacuum pump prevailed in the vacuum processing chamber to cause the pressure of the vacuum processing chamber of the film forming apparatus to disadvantageously increase. A high vacuum pump is required to the vacuum processing chamber so as to avoid this problem. When the evacuating passage has relatively large cross section so as not to considerably increase the pressure difference, there is a problem that it is difficult to sufficiently remove the dusts from the evacuated gas.
As described above, since it is not compatible between to sufficiently remove the dusts and to reduce the pressure difference, it must be comprised at a certain point. As a result, since the necessary pressure difference cannot be considerably reduced, it might be sometimes difficult to install the dust collection unit ( trap ) between a middle vacuum pump ( e.g., Roots vacuum pump ). Because, when the trap having large pressure difference is installed between the middle vacuum pump and the vacuum chamber, the pumping of the middle vacuum pump does not effectively operate in the vacuum chamber due to the pressure difference in the trap.
Therefore, due to the face that the pressure difference necessary to pass the gas through the dust collector cannot be sufficiently reduced, there arises a difficulty that high vacuum condition cannot be obtained without deteriorating the performance of the vacuum pump used in the vacuum evacuating system of the film forming apparatus. Further, when oil is used to remove the dusts, there is also a problem that oil component may flow into the vacuum chamber to adversely influence the film forming apparatus.
With the ultrafine particle producing apparatus, the produced ultrafine particles are intaken together with the evacuated gas by the vacuum pump to cause defects to deteriorate the efficiency for collecting these particles.
It is, therefore, an object of this invention to solve the drawbacks of the above-mentioned conventional apparatuses and to provide a fine particle collector trap for a vacuum evacuating system which can effectively collect fine particles such as dusts or the like in low pressure gas without raising the pressure difference between a vacuum processing chamber and the evacuating system.
Another object of the invention is to provide a fine particle collector trap for a vacuum evacuating system which makes it possible to use a vacuum pump obtaining relatively high vacuum with ready manufacture and provide easy maintenance.
A further object of the invention is to provide a fine particle collector trap for a vacuum evacuating system which may be utilized for collecting ultrafine particles produced in an ultrafine particle producing apparatus.